The present invention relates to an isolation valve with a dead chamber.
In the petroleum field, it is known how to apply isolation valves with a dead chamber on pipings conveying hydrocarbons for example. Such valves actually give the possibility of isolating a segment of a pipeline in a leak-proof way, thereby giving the possibility of stopping the supply to a fire, of avoiding a hydrocarbon leak, ensuring maintenance on the pipeline or perfectly segregating various products by avoiding any cross-contamination. This type of isolation valve is particularly interesting because it ensures a dual seal:                between the upstream area and the dead chamber, on the one hand; and        between the dead chamber and the downstream area, on the other hand.        
An example of an isolation valve with a dead chamber is illustrated in FIG. 1. In this case, the illustrated isolation valve 10 is of the ball type. Such an isolation valve conventionally comprises a body 12 with a fluid-passing conduit 14, a ball 16, and a stem 18 for actuating the ball 16, giving the possibility of actuating the ball 16 between a fluid-passing position through the conduit 14 (position illustrated in the figure) and a position for obturating the conduit 14. The actuation of the ball 16 here consists in rotating by a quarter of a turn the ball 16 around the axis of the stem 18.
The isolation valve 10 of FIG. 1 also comprises upstream 20 and downstream 22 seats sealably bearing against the ball 16 around the fluid-passing conduit 14, on the one hand and against the body 12, on the other hand. It is known that in such an isolation valve 10, the ball has a cavity 32 which forms, in the obturation position of the fluid-passing conduit, together with the body 12, a dead chamber. This dead chamber contains fluid—a hydrocarbon in the application mentioned hereinbefore—confined in the cavity 32, at the moment when the spherical ball 16 passes from its fluid-passing position to its position for obturating the fluid-passing conduit 14.
Now, this type of valve may be exposed to variations in temperature. An increase in the temperature generates an expansion of the liquid confined in the dead chamber, which, because of the incompressibility of the latter, generates a rise in pressure in the dead chamber. This rise in pressure may, in extreme cases, lead to bursting of the isolation valve. For example, the expansion of a hydrocarbon is of the order of 0.1% of the volume per degree Celsius which, by neglecting the expansion of the body of the isolation valve due to its thermal expansion and due to the rise in pressure and considering a perfect seal, leads to an increase in pressure of the order of 10 bars/° C.
In order to reduce the pressure in the dead chamber, it is known how to design this type of isolation valve with a device giving the possibility of decompressing the dead chamber. This device may consist in seats adapted for moving away from the ball in the case of overpressure in the dead chamber. However, such seats are detrimental to the seal of the isolation valve. Alternatively, the decompression device may be positioned outside the body and be connected to the dead chamber through a conduit crossing the body.
However, for safety reasons, the isolation valve should, when it is subject to a fire:                not leak in a line towards the downstream area, for ensuring the isolation;        not leak towards the outside of the isolation valve, in order to avoid feeding the fire in which the valve is found; and        allow control of the increase in pressure inside the isolation valve and notably inside the dead chamber in order to avoid failure of the isolation valve.        
It is found that applying a decompression device outside the body creates vulnerability of the isolation valve, notably in the case of a fire since the latter is then exposed to flames and to high temperatures. Such a solution therefore does not give the possibility of ensuring the fire resistance criteria mentioned above. Therefore there exists a need for an isolation valve with a dead chamber having increased fire resistance.
Further, FR-A-2 432 661 relates to a hydraulic distributor device comprising a ball plug valve. Document EP-A-2 423 549 relates to a safety discharge device for a two-way valve. Document GB-A-2,226,385 relates to a spherical ball valve.
Moreover, document GB-A-1,346,357 relates to a valve with a spherical rotating body comprising, in a wall of the spherical rotating body, a passage for circulation of fluid obstructed by a pressure discharge device. The pressure discharge device comprises an element directly flattened against the wall of the rotating body by means of a spring itself bearing against a plug screwed into the passage of the spherical rotating body. The pressure discharge device is adapted, under the effect of an increase in pressure inside the spherical rotating body, for moving the element in order to clear the passage formed in the wall of the rotating body. The valve with a spherical rotating body of document GB-A-1,346,357 has the drawback of preventing any adjustment of the discharge device prior to its mounting on the valve with a spherical rotating body so that the mounting of the pressure discharge device in the passage of the spherical rotating body interferes with the adjustment of the spring. Further, the configuration of the spherical rotating body only allows limited passing of fluid through the passage causing significant drops in pressure on the one hand and a risk of failure in the case of a significant increase in pressure, for example during a high heat input.
For this purpose, the present invention proposes an isolation valve with a dead chamber, including a body with a fluid-passing conduit, a device for obturating the fluid passage in the conduit, which may be actuated between a fluid-passing position and a position for obturating the fluid passage in the conduit, an obturation position in which the obturation device forms a dead chamber, a wall of the obturation device comprising a through-hole, in which is positioned a device for decompression of the dead chamber, the obturation device including a tapered rotating plug. According to preferred embodiments, the invention comprises one or several of the following features:                the decompression device is formed with a calibrated valve;        the calibrated valve is screwed, pinned or force-fitted into the wall of the obturation device;        the through-hole is made between the dead chamber and the fluid-passing conduit;        the through-hole is made in the wall of the obturation device intended to be positioned upstream from the isolation valve, in a position for obturating the passage for fluid in the conduit;        the rotating body also comprises two sliders translationally mounted on the tapered plug in the body, for example by means of a dovetail, so that a translation of the tapered plug along its axis induces translation of both sliders along a direction substantially perpendicular to the direction of the axis of the tapered plug;        the device for decompression of the dead chamber is positioned in one of the two sliders;        the tapered plug has a shaft for guiding the rotation of the rotating body.        
The invention also relates to a hydrocarbon transport facility including at least two pipes connected through an isolation valve with a dead chamber as described hereinbefore, in all of its combinations. Other features and advantages of the invention will become apparent upon reading the description which follows of preferred embodiments of the invention, given as an example and with reference to the appended drawings.